Modern automotive vehicles use a vapor storage device to collect fuel vapor that would otherwise simply be vented from the storage fuel tank and from the fuel system. The fuel tank produces some fuel vapors by diurnal cycling. An even greater volume of fuel vapor is produced as the vehicle operates, the so called running losses. When fuel sent from the fuel tank and not burned in the engine is returned to the fuel tank, it is warmer, especially in vehicles using fuel injection systems, and is permeated with many small fuel vapor bubbles. The return of fuel in this condition accelerates fuel vapor formation in the tank. An even greater volume of fuel vapor is displaced whenever the tank is filled, and future regulations will require that this, too, be collected, rather than being vented. Therefore, any means that could reduce fuel vaporization in the tank and free up vapor storage capacity would be of great use.
Most vehicles have a fuel handling system to assure a steady supply of fuel from the tank to the engine. Typically, some kind of reservoir canister is used to assure a continual supply of fuel to the inlet of the fuel pump, avoiding the temporary fuel starvation that could be caused by fast cornering or low fuel. With fuel injection systems, such systems often include a two stage pump. A first stage pump sends fuel directly from the tank into the reservoir canister, while a second stage, higher pressure pump sends fuel from the canister through the fuel rail of the engine. Some systems also route the return fuel back to the canister, to help keep the canister filled, and provide an outlet to the fuel tank to let fuel vapor escape the canister. While such systems assure a supply of fuel for the second stage pump, they generally do nothing to reduce running loss, and even increase it. The reservoir canisters typically have overflow openings back into the main tank, so heated return fuel can mix with the fuel in the main tank, raising the temperature of the whole tank and increasing the rate of fuel vaporization. Some systems even use the flow of the return fuel to run a jet pump that actively forces more fuel from the main tank back into the canister, with the excess running out the top and back into the tank.
Fuel handling systems in the past have not been concerned with reducing running losses, only with assuring fuel supply and efficient fuel pump operation. An exception is the system disclosed in U.S. Pat. No. 4,989,572, issued Feb. 5, 1991, and assigned to the assignee of the subject invention. Hot return fuel is routed to a reservoir canister, but mixing of the return fuel with the main fuel store is substantially prevented. The reservoir canister has an internal pump, and is closed to the main tank, except for a vapor outlet into the main tank and a one-way make-up fuel inlet in the form of a flapper door. The flapper door opens easily to let cold make-up fuel in from the main tank when the hot return fuel alone is not adequate to meet engine demand. However, the flapper door shuts just as easily to stop substantially all of the hot return fuel from running out of the canister and back into the tank. While the system is very effective in reducing running losses, it may be unsuitable for vehicles with a high fuel demand engine. That is, make-up fuel from the main tank is supplied only passively, through the swinging door, rather than being actively forced in.